1. Field of the Invention
The present disclosure relates to a water content evaluation method and a water content evaluation apparatus which measures water content contained in a leaf or a part of plant.
2. Description of the Related Art
There is a potential difference inside and outside of a cell in a normal plant and electromotive force is generated. It is possible to describe a mechanism which generates such electromotive force based on, for example, an electrophysiological model of an axial organ of a higher plant. In particular, various methods are suggested in which a state of a root of the plant (for example, water stress) is examined non-destructively utilizing electromotive force between the root and soil.
As a technique in which water stress in a plant is measured utilizing the method described above, for example, JP-A-2001-272373 discloses connecting a first nonpolarizable electrode to the plant, connecting a second nonpolarizable electrode to soil in which the plant is planted, providing a potentiometer between the two nonpolarizable electrodes, and being able to measure water stress which is received by the plant by measuring electromotive force between both nonpolarizable electrodes using the potentiometer.
In a case where water content that is contained in a leaf is measured by irradiating the leaf of the plant with a near infrared beam and status of the plant is evaluated, the configuration in PTL 1 has the following problem. The leaf of the plant performs movement such as contraction by bending or coiling, and opening or closing in the morning, in the afternoon, and in the evening, daily or in increments of time.
In a case where water content is measured which is contained in the leaf by irradiating the leaf of the plant with the near infrared beam and status of the plant is evaluated, the leaf of the plant performs movement such as contraction by bending or coiling, and opening or closing in the morning, in the afternoon, and in the evening, daily or in increments of time. Thickness of the leaf changes in an optical axis direction according to the movement of the leaf. For example, when angle θ is inclined to the front due to the leaf that stands in a vertical direction with respect to the optical axis bending, the thickness of the leaf in the optical axis direction is raised by (1/cos θ). The increase in the thickness is obtained by a measurement result in which the water content that is contained in the leaf is greater than in reality. In addition, when measuring by irradiating a front surface of the leaf with the near infrared beam at a predetermined spot diameter, the leaf is damaged from a portion of an irradiation range due to movement of the leaf, an irradiation area (projection area) of the leaf is small, and the measurement result is obtained in which the water content is lower than in reality.
In this manner, an error may be generated in measurement precision of the water content of the leaf, and it may not be possible to correctly evaluate status of the plant since accurate water content is not obtained.
In addition, the leaves of a seedling in a field grow in abundance and are foliage. In the foliage, a plurality of leaves overlap is respective orientations, and for example, when wind blows, the leaves relatively move.
Since the water content which is contained in the leaf is measured by irradiating the leaf of the plant with the near infrared beam and status of the plant is evaluated, in a case where the front surface of the leaf of the plant is irradiated with two types of near infrared beams and water content is obtained from a reflection intensity rate therefrom, the radiated near infrared beam is absorbed and scattered due to the leaf on a periphery of a leaf that is a measurement target (refer to FIG. 21A). Other than, for example, the radiated near infrared beam being absorbed by the leaf that is the measurement target, a leaf on the left side is also radiated and a portion is absorbed. The leaf on the left side is radiated, and the near infrared beam that is scattered by the leaf on the left side is diffused on the leaf that is the measurement target. In addition, multiple scattering also occurs in which a leaf on the right side is radiated, and diffused light that is scattered by the leaf on the right side is diffused to another leaf and is diffused on the leaf that is the measurement target. Since the background of the reflection intensity rate that is obtained by measurement is significantly raised, the multiple scattering is difficult to distinguish individually to the leaf that is the measurement target and the leaves on the periphery of the measurement target. In addition, the plurality of leaves overlap or are separated, and a target area of the leaf on which the near infrared beam is radiated changes (refer to FIG. 21B).
Accordingly, even in measurement of presence or absence of water content, individual leaves on the periphery and the target leaf are difficult to distinguish.